This invention relates to a static laminar mixing device and to an admixing device for a static laminar mixing device. More particularly, this invention relates to a static laminar mixing device for mixing media of different viscosities.
Heretofore, various types of static laminar mixing devices have been constructed for the mixing of flowable media. Generally, these devices have been built up of the so-called static mixer elements in which the flow of the media to be mixed, e.g., liquid with liquid or liquid with gas, remains laminar. Laminar mixers of that kind are employed in mixing devices for the mixing of, e.g., liquids of widely different viscosities, such as in the admixing of low-viscosity soluble additives to high-viscosity liquids.
During mixing, the low-viscosity (limpid) liquid is usually fed to the main flow of the high-viscosity (viscid) liquid, e.g., via a tube which may be arranged before or directly at the inlet to the mixer element and opens into the main flow of the high-viscosity liquid.
However, when the viscosities of the liquids to be mixed differ by orders of magnitude (difference 5xc3x97103 to 106 or more), in order to achieve adequate solution and/or thorough mixing, relatively long lengths of mixer are necessary in the mixing devices.
Other mixing devices have also been known for mixing fluids having different viscosities such as described in Japanese Patent Application No. 62-191274 and Japanese Patent Application No. 57-15258. However, the apparatus used for mixing has either been of the conventional static mixer type or of a rather cumbersome convoluted mixer type.
Still other mixing apparatus employing static mixer elements have been known from U.S. Pat. No. 4,255,125, U.K. Patent Application 2,010,739 and French Patent 2,223,073.
Mixing devices wherein an additive is introduced via a nozzle or the like have also been described in U.S. Pat. Nos. 4,073,479 and 3,770,208 as well as German OS 2 320 609.
Accordingly, it is an object of the invention to improve the efficiency of a static laminar mixing device for the mixing of media with different viscosities.
It is another object of the invention to reduce the space required for the mixing of flowable media within static laminar mixing devices.
It is another object of the invention to improve the mixing results of mixing two media of greatly different viscosities.
Briefly, the invention provides a static laminar mixing device which is comprised of a mixer having an inlet for receiving at least a first flow of high viscosity medium and a plurality of static mixer elements disposed along a longitudinal axis thereof for mixing media of different viscosities together and an admixing device for introducing a second flow of low viscosity medium into the inlet of the mixer. This admixing device includes a plate transverse to the flow of the first medium and having at least one convergent orifice therein for passage of the first medium therethrough into the mixer as well as a duct adjacent the plate for passage of the second medium therefrom into the orifice.
The mixing device may also have a second mixer coaxial of the first mixer for receiving media therefrom wherein this second mixer has a cross-sectional area through which the media flow (cross-sectional flow area) which is greater than the cross-sectional flow area of the first mixer. The second mixer also has a plurality of static mixer elements disposed along a longitudinal axis for receiving and mixing the media of different viscosities together. In this embodiment, the cross-sectional flow area of the first mixer is in a ratio relative to the cross-sectional flow area of the second mixer of less than or equal to 1:2.
In still another embodiment, the static laminar mixing device may be constructed of two mixers, each of which has a plurality of static mixer elements, as above, with an admixing device of generally conventional structure for introducing the flow of low viscosity medium into the inlet of the first mixer. In this embodiment, the cross-sectional flow areas of the two mixers are in a ratio wherein the second mixer is at least twice as large as the first mixer.
With the same total number of mixer elements in two mixers of different cross-sectional flow areas, the laminar mixing device cannot only achieve a mixing of improved thoroughness but also a considerably improved solution of the admixed liquid or respectively of an admixed gas in a high-viscosity liquid of the main flow.
By way of example, during operation of a conventional laminar mixing device for polystyrene, which exhibits one single mixing column about 1.5 meters (m) long and is provided with thirty mixer elements of SULZER TYPE SMX DN50, only 1 to 2% of mineral oil/paraffin oil additive becomes dissolved in the polystyrene. However, under the same conditions, a laminar mixing device constructed as above with two mixers of different cross-sectional flow areas according to the invention, 4 to 6% of the additive may be observed to be dissolved and, in the case of simultaneous employment of the admixing device noted above still more. In this case, the mixing device exhibits a first mixing column, also called the pre-mixer, provided with twelve mixer elements of SULZER type SMX DN17, and a second mixing column, also called the main mixer provided with eighteen mixer elements of SULZER type SMX DN50. The two mixing columns of the mixing device exhibit together a length of merely 1.1 meters (m) with considerably improved, at least doubled admixture of the additive. The number of mixer elements in the first mixing column should be at least four but less than half the total number of mixer elements in the two mixing columns.
The admixing device with the convergent orifice-plate in the main flow of the high-viscosity liquid, which is arranged in the region of the mouth of the feed duct for the low-viscosity medium, can raise the maximum possible homogeneously immiscible amount of the additive or low-viscosity medium by up to about one third in comparison with conventional admixing devices or metering devices. By the introduction of the convergent orifice-plate, there results even in the case of laminar flow conditions an improved solution of smaller drops of the low-viscosity component from the feeder nozzle.
The cross-section of mixing columns is practically always circular and the diameter of the orifice in the likewise appropriately circular orifice-plate should as a rule be at most ⅔ the diameter of the main flow, i.e. of the mixing column.
An admixing device, also called the metering station, may also comprise a number of orifice-plates which are arranged side by side. In this case, a number of orifice-plates with feed ducts for the additive would be distributed over the cross-section of the main flow. The total cross-sectional areas of the several orifice-plate openings should, in this case, advantageously be less than half the cross-sectional areas of the main flow. It has to be ensured that the flow even in the region of the convergent orifice-plates is still laminar.
The employment of the admixing device (metering device) of the kind described is in itself already advantageous in its employment with a mixing column. In combination with an improved laminar mixing device according to the invention, the productive capacity of the metering device becomes particularly effective in support of the increased mixing capacity of the mixing device.
The speeds of flow in the premixer and main mixer lie typically in the range from one to one hundred millimeters per second (mmsecxe2x88x921), for example, from about 50 millimeters per second (mmsecxe2x88x921) in the premixer or 1 to 10 millimeters per second (mmsecxe2x88x921) in the main mixer.
Both with the laminar mixing device alone and with the admixing device alone but particularly in combination, distinctly improved mixing results are achieved. In the admixture of additives, such as mineral oil/paraffin, to and their dissolving in plastics melts such as polystyrene melts, outstanding results are achieved by the mixing device.